Hard disk drives (HDDs) using rotating media are a low-cost and robust solution for permanent storage of data. Current generations of HDDs exist in different form factors, with the 2.5″ form factor prevailing in the mobile sector and the 3.5″ form factor being the most common solution in desktop, server and workstation environments. The system interface of either of the above form factors may conform to the Serial ATA (SATA) or the Serial Attached SCSI (SAS) standard with the first being prevalent in the consumer market segment and the latter more commonly used in the enterprise market segment.
Contemporary high density storage servers are typically modular designs having a drive enclosure and provisions to plug in additional modules into a server or storage bay. The modules are typically server modules or I/O modules that interface with the outside world through SAS connections. The modules are also typically field-replaceable units (FRUs), whereas the drive enclosure is the main unit of the entire configuration and uses a base board (also referred to as base plane or drive plane) as an interface to the drives. The drives are inserted from the top of the enclosure, which is referred to as top-loading configuration.
FIG. 1 illustrates a top plan view of a typical base board 100 as used in current high density servers. The base board 100 includes a printed circuit board 110 having a plurality of female SAS receptacles 115. The printed circuit board 110 may be connected to a compute node, such as a server or I/O module (IOM), for example, via signal connectors that are connected to an I/O controller (IOC) or, more commonly, to a SAS expander on the server or IOM. In addition to the plurality of drive connectors 115, the printed circuit board 110 may also include a plurality of system interface connectors 150, one or more power connectors 170 for connection to the server and/or I/O modules and one or more bus bar tie downs 190.
A commonly used example for the signal connector is the FCI AirMax connector 150. In many current designs, the server or IOM is not directly connected to the power supply of the system but instead receives power through the drive base board via a dedicated power connector 170. In some cases, the base board 110 receives most of the power from the system power supplies via dedicated power connectors, or alternatively, power bus bars that are tied into dedicated bus-bar tie downs.
Storage drives, regardless of whether they are SATA or SAS drives are plugged into the female SAS receptacles 115, compatible with either SAS or SATA drives. These receptacles are soldered onto the base board 100 and connect with the necessary high speed differential signal pairs for transfer and receive (Tx and Rx, respectively) to SAS expanders on the compute or I/O node of the server. Power to the drives is delivered to the drives through dedicated pins on the same connectors.
FIG. 2 illustrates a perspective view of a typical base board 200 with a HDD 205 inserted into a drive connector 215. As shown, the base board 200 includes a printed circuit board 210 having a plurality of drive connectors or female SAS receptacles 215. The printed circuit board may be connected to a server or I/O module (IOM), for example, via signal connectors that are connected to an I/O controller (IOC) or, more commonly, to a SAS expander on the server or IOM. In addition to the plurality of drive connectors 215, the printed circuit board 210 may also include a plurality of system interface connectors 250, one or more power connectors 270 for connection to the server and/or I/O modules and one or more bus bar tie downs 290. For ease of understanding, a 3.5″ form factor HDD 205 is shown inserted into one of the available drive connectors.
In the case of SAS drives, signal speed has approached 12 Gb/s, especially in the case of larger enclosures which pose serious problems with respect to maintaining signal integrity across the entire base board. In fact, in many cases, it is not possible to maintain 12 G capability through the entire depth of the base board. Consequently, only a few drives in the locations most proximal to the expander connection are capable of running at the 12 G protocol whereas drives on connectors that exceed a given trace length for their interconnect have to be scaled back to run at a slower speed. Typically drives are scaled back to 6 Gb/s but in some cases, based on the dielectric properties of the circuit board substrate and the length of the traces, signal integrity issues may force the drives to run at 3.0 Gb/s host transfer rate.
In the case of a server using spindle-based hard disk drives (HDDs), 6 Gb/s transfer rates are acceptable, however, a large portion of the storage market is rapidly embracing solid state drives (SSDs) that natively support 12 Gb/s transfers. In this case, the base board or base plane becomes the limiting factor for the operation of the drive.
Better signal integrity can be achieved by using high speed materials, such as iSpeed or IT-150DA laminate, as well as through optimization of trace geometry, that is trace length matching and phase alignment. However, high speed materials which minimize signal loss are expensive and trace optimization requires a significant amount of extra work, especially if traces have to be routed through several layers connected by vias and if the latter have to be manually back-drilled for impedance matching.
In combination, all the above mentioned results in increased cost for high speed base boards. Additionally, different server configurations will require different base boards, each of which will have to be designed from scratch since there is no possibility for carrying over only parts of an existing design without compromising signal integrity.
In view of the aforementioned problems relating to functional enabling high speed signaling on high density server enclosures, it is clear that alternative solutions are necessary.